This invention relates to electrically programmable and erasable memory devices of the type employing hot electron injection.
Electrically programmable and erasable memory devices employing hot electron injection are known. An example of such a device is described in U.S. Pat. No. 4,087,795. Briefly, such devices include a floating gate surmounted by a conductive control gate and separated therefrom by an insulator. In such devices, programming is achieved by means of the avalanche effect in accordance with which the floating gate is charged with carriers that are heated, or have relatively greater energy levels, as a result of the avalanche breakdown at the interface between the drain and the substrate, upon the breakdown of this otherwise non-conducting p-n junction. Due to their increased energy, these carriers can pass through the insulator, particularly if a voltage accelerating them is applied between the substrate and the floating gate. In such drain side channel hot electron injection devices, the injection current can be enhanced by increasing either the channel field or the gateoxide field. However, low gate and high drain voltages are required to generate the high channel field, while the exact opposite biasing condition--i.e., low drain and high gate voltages--is needed to create the high gate-oxide field. Because of these conflicting conditions, optimization of the performance of such devices is difficult to achieve. In practice, very high drain and gate voltages are typically used as a compromise: however, under such voltage biasing, a typical device is subjected to biasing conditions which are very close to the breakdown voltages. As a result, there exists an extremely small tolerance window for device design and process control.
A further disadvantage inherent in drain-side channel hot-electron injection devices is the relatively low hot-electron injection efficiency (defined as the ratio of the number of electrons injected into the floating gate to the number of electron-hole pairs generated in the device channel), which is lower than 10.sup.-7 and which severely limits the maximum programming speed. Conventional attempts to increase the programming speed typically involve scaling down the physical dimensions of the devices: however, a reduction in physical dimensions is usually accompanied by a decrease in device yield during batch processing and a deterioration in the ability of the devices to maintain their programmed or erased state over time.
A still further disadvantage with drain-side channel hot-electoon injection devices resides in the fact that the relatively high drain and gate voltages used to bias such devices during programming typically necessitates use of at least one separate power supply in order to generate the programming and erasing voltages, which are not otherwise required for the operation of circuits employing such devices.